The present invention relates generally to cutting elements for use in earth-boring drill bits and, more specifically, to a means for increasing the life of cutting elements that comprise a layer of superhard material, such as diamond, affixed to a substrate. Still more particularly, the present invention relates to a polycrystalline diamond enhanced insert comprising a supporting substrate and a diamond layer supported thereon.
In a typical drilling operation, a drill bit is rotated while being advanced into a soil or rock formation. The formation is cut by cutting elements on the drill bit, and the cuttings are flushed from the borehole by the circulation of drilling fluid that is pumped down through the drill string and flows back toward the top of the borehole in the annulus between the drill string and the borehole wall. The drilling fluid is delivered to the drill bit through a passage in the drill stem and is ejected outwardly through nozzles in the cutting face of the drill bit. The ejected drilling fluid is directed outwardly through the nozzles at high speed to aid in cutting, flush the cuttings and cool the cutter elements.
The present invention is described in terms of cutter elements for roller cone drill bits, although its benefits can be realized in percussion bits as well as other fixed cutter bits. In a typical roller cone drill bit, the bit body supports three roller cones that are rotatably mounted on cantilevered shafts, as is well known in the art. Each roller cone in turn supports a plurality of cutter elements, which cut and/or crush the wall or floor of the borehole and thus advance the bit.
Conventional cutting inserts typically have a body consisting of a cylindrical grip portion from which extends a convex protrusion. In order to improve their operational life, these inserts are preferably coated with a superhard, sometimes also known as ultrahard, material. The coated cutting layer typically comprises a superhard substance, such as a layer of polycrystalline diamond, thermally stable diamond or any other ultrahard material. The substrate, which supports the coated cutting layer is normally formed of a hard material such as tungsten carbide (WC). The substrate typically has a body consisting of a cylindrical grip from which extends a convex protrusion. The grip is embedded in and affixed to the roller cone and the protrusion extends outwardly from the surface of the roller cone. The protrusion, for example, may be hemispherical, which is commonly referred to as a semi-round top (SRT), or may be conical, or chisel-shaped or may form a crest that is inclined relative to the plane of intersection between the grip and the protrusion. The latter embodiment, along with other non-axisymmetric shapes, is becoming more common, as the cutter elements are designed to provide optimal cutting for various formation types and drill bit designs.
The basic techniques for constructing polycrystalline diamond enhanced cutting elements are generally well known and will not be described in detail. They can be summarized as follows: a carbide substrate is formed having a desired surface configuration; the substrate is placed in a mold with a superhard material, such as diamond powder and/or a mixture of diamond with other material that forms transition layers, and subjected to high temperature and pressure, resulting in the formation of a diamond layer bonded to the substrate surface.
Although cutting elements having this configuration have significantly expanded the scope of formations for which drilling with diamond bits is economically viable, the interface between the substrate and the diamond layer continues to limit usage of these cutter elements, as it is prone to failure. Specifically, it is not uncommon for diamond coated inserts to fail during cutting. Failure typically takes one of three common forms, namely spalling/chipping, delamination, and wear. External loads due to contact tend to cause failures such as fracture, spalling, and chipping of the diamond layer. The impact mechanism involves the sudden propagation of a surface crack or internal flaw initiated on the PCD layer, into the material below the PCD layer until the crack length is sufficient for spalling, chipping, or catastrophic failure of the enhanced insert. On the other hand, internal stresses for example, thermal residual stresses resulting from manufacturing process, tend to cause delamination of the diamond layer, either by cracks initiating along the interface and propagating outward, or by cracks initiating in the diamond layer surface and propagating catastrophically along the interface. Excessively high contact stress and high temperature, along with a very hostile downhole operation environment, are known to cause severe wear to the diamond layer of cutting elements in percussion bits. The wear mechanism occurs due to the relative sliding of the PCD relative to the earth formation, and its presence as a failure mode is related to the basic bit type, abrasiveness of the formation, as well as other factors such as formation hardness or strength, and the amount of relative sliding involved during contact with the formation. Wear is not a typical failure mode in roller cone drill bits that utilize conventional diamond coated cutting elements. Instead, fatigue and impact of the diamond coating are the typical failure modes found.
One explanation for failure resulting from internal stresses is that the interface between the diamond and the substrate or a transition layer is subject to high residual stresses resulting from the manufacturing processes of the cutting element. Specifically, because manufacturing occurs at elevated temperatures, the differing coefficients of thermal expansion of the diamond and substrate material result in thermally-induced stresses as the materials cool down from the manufacturing temperature. These residual stresses tend to be larger when the diamond/substrate interface has a smaller radius of curvature. At the same time, as the radius of curvature of the interface increases, the application of cutting forces due to contact on the cutter element produces larger debonding and other detrimental stresses at the interface, which can result in delamination. In addition, finite element analysis (FEA) has demonstrated that during loading, high stresses are localized in both the outer diamond layer and at the diamond transition-layer/tungsten carbide interface. Finally, localized loading on the surface of the inserts causes rings or zones of tensile stress, which the PCD layer is not capable of handling.
In drilling applications, the cutting elements are subjected to extremes of temperature and heavy loads when the drill bit is in use. It has been found that during drilling, shock waves may rebound from the internal planar interface between the two layers and interact destructively.
All of these phenomena are deleterious to the life of the cutting element during drilling operations. More specifically, the residual stresses, when augmented by the repetitive stresses attributable to the cyclical loading of the cutting element by contact with the formation, may cause spalling, fracture and even delamination of the diamond layer from the substrate. In addition to the foregoing, state of the art cutting elements can lack sufficient diamond volume to cut highly abrasive formations, as the thickness of the diamond layer tends to be limited by the resulting high residual stresses and the difficulty of bonding a relatively thick diamond layer to a curved substrate surface. For example, even within the diamond layer, residual stresses arise as a result of temperature changes. Because these stresses typically increase as the thickness of the layer increases, this factor tends to be viewed as limiting on thickness.
Hence, it is desired to provide cutting elements that provide increased fatigue life, and/or impact resistance and/or wear resistance without increasing the risk of spalling or delamination.
The present invention provides a diamond cutting element with increased life expectancy. The improved cutting element has an optimized substrate/coating interface and incorporates a region of exceptional thickness in its cutting layer. This region of thicker diamond on the cutting element is oriented so that it is the primary cutting surface and sustains the major loading while cutting the rock formation. The improved diamond cutting element has several advantages. One advantage is that the exceptionally thick diamond region is stronger and more rigid, which significantly reduces localized deformation under loading. When the localized deformations are reduced, the associated Hertzian tensile stresses are reduced, which ultimately reduces or eliminates chipping and breaking of the diamond coating. Another advantage of the stronger, more rigid diamond layer region is that it reduces the bending stresses at the substrate/coating interface when the cutting surface is loaded, which reduces the potential for coating debonding and/or breakage. Yet another advantage is that substrate/coating interface is farther away from the loaded cutting surface of the cutter element, therefore keeping the maximum shear stresses away from the substrate/coating interface, which is typically a relatively weak part of a diamond coated cutter element. Still yet another advantage is that because the cutter element has thicker, greater volume of diamond on the cutting surface, a tougher diamond grade can be utilized. Generally, a diamond grade that has increased toughness over another grade also has less wear resistance, thus the increase in the volume of diamond material to wear away is beneficial. If an increase in toughness is not required, the overall wear resistance of the cutter element is improved just through the increased volume in the diamond in the contact region.
The present cutter element compensates for the resulting residual stresses that might otherwise be caused by a region of exceptional thickness by providing an interface geometry that balances the reduction in bending stresses associated with the region of increased thickness with the increase in interface delamination stresses resulting from a decreased radius of curvature. The interface is designed so that even without transition layers or a non-planar interface, the residual stress due to thermal mismatch is still minimized. More specifically, the present cutter element provides a region of exceptional thickness that has a preferred volume ratio to the volume of the cutting layer and provides a cutting layer that has a preferred volume ratio to the volume of the protrusion portion of the cutter element.
The region of exceptional thickness can be defined in the present invention in terms of volume ratios of the cutting layer in various regions of the cutting surface, or can alternatively be defined in terms of the configurations of the substrate and cutting layers. In each instance, one objective of the present invention is to provide a variation in cutting layer thickness, so that the cutting layer in the region of the cutter element that is expected to receive the most wear is thicker than in other portions of the cutting surface.
In one embodiment, a cutter element for use in a drill bit comprises a substrate comprising a grip portion and an extension portion, where the grip portion has an insert axis and the extension portion has a substrate apex. A superhard cutting layer is affixed to the extension portion. The cutting layer covers the substrate apex and defines an interface surface on the extension portion, the interface surface being free of edges underneath the cutting layer, and the cutting layer having a cutting surface that defines a cutting apex. The cutting layer and extension portion are shaped such that a plane can be passed through the insert axis to divide the cutting layer where the volume of the cutting layer on a first side of the plane is at least 60 percent of the total volume of the cutting layer.
In another embodiment, a cutter element for use in a drill bit comprises a substrate comprising a grip portion and an extension portion, the grip portion having an insert axis and the extension portion having a substrate apex, and a superhard cutting layer affixed to the extension portion to define an interface surface on the extension portion and having a cutting surface, wherein the cutting layer and the extension portion are shaped such that a plane can be passed through the insert axis to divide the cutting layer such that the volume of cutting layer on one side of the plane is at least 60 percent of the total volume of the cutting layer and wherein the cutting surface is axisymmetric.
In still another embodiment, a cutter element for use in a drill bit comprises a substrate comprising a grip portion and an extension portion, the grip portion having an insert axis and the extension portion having a substrate apex. A superhard cutting layer is affixed to the extension portion to define an interface surface on the extension portion. The cutting layer has a cutting surface. The cutting layer and the extension portion are shaped such that a plane can be passed through the insert axis to divide the cutting layer such that the volume of cutting layer on one side of the plane is at least 60 percent of the total volume of the cutting layer and wherein the cutting surface is free of cutting edges.
In still another embodiment, a cutter element for use in a drill bit comprises a substrate comprising a grip portion and an extension portion, the grip portion having an insert axis and the extension portion having a substrate apex. A superhard cutting layer is affixed to the extension portion to define an interface surface on the extension portion. The cutting layer has a cutting surface defining a cutting apex. The cutting layer and the extension portion are shaped such that a plane can be passed through the insert axis to divide the cutting layer such that the volume of the cutting layer on a first side of the plane is at least 75 percent of the total volume of the cutting layer.
In still another embodiment, a cutter element for use in a drill bit comprises a substrate comprising a grip portion and an extension portion, the grip portion having an insert axis and the extension portion having a substrate apex. A superhard cutting layer is affixed to the extension portion so as to define an interface surface on the extension portion. The cutting layer has a cutting surface defining a cutting apex that is offset from the substrate apex, the cutting layer covering the substrate apex. The substrate and the cutting layer are shaped such that the insert axis does not pass through the substrate apex, and a plane parallel to the insert axis can be passed through the substrate apex to divide the cutting layer such that the volume of the cutting layer on a first side of the plane is at least 75 percent of the total volume of the cutting layer.
In still another embodiment, a cutter element for use in a drill bit comprises a substrate comprising a grip portion and an extension portion, the grip portion having an insert axis and the extension portion having a substrate apex. A superhard cutting layer is affixed to the extension portion, the cutting layer covering the substrate apex. The substrate and the cutting layer are shaped such that a plane parallel to the insert axis and passing through the first apex divides the cutting layer such that the volume of the cutting layer on a first side of the plane is at least 60 percent of the total volume of the cutting layer and the cutting surface is axisymmetric.
In another embodiment, a cutter element for use in a drill bit comprises a substrate comprising a grip portion and an extension portion, the grip portion having an insert axis and the extension portion having a substrate apex. A superhard cutting layer is affixed to the extension portion. The substrate and the cutting layer are shaped such that a plane parallel to the insert axis and passing through the first apex divides the cutting layer such that the volume of the cutting layer on a first side of the plane is at least 60 percent of the total volume of the cutting layer and the cutting surface is free of cutting edges.
Another cutter element for use in a drill bit comprises a substrate comprising a grip portion and an extension portion, said grip portion having an insert axis, the extension portion having a volume Vext. A superhard cutting layer is affixed to the extension portion so as to define an interface surface on the extension portion and having a cutting surface defining a cutting apex, the entire cutting layer having a volume Vcl. The extension portion and the cutting layer are configured such that a plane P* can be passed through the insert axis such that the ratio of the volume of the cutting layer on a first side of the plane P* to the total volume on the first side of the plane (Vclxe2x88x921*: (Vextxe2x88x921*+Vclxe2x88x921*)) is at least 60 percent and less than 98% and the same ratio (Vclxe2x88x921*: (Vextxe2x88x921*+Vclxe2x88x921*)) is greater than a corresponding ratio on a second side of the plane (Vclxe2x88x922*: (Vextxe2x88x922*+Vclxe2x88x921*)) and the volume on the first side of the plane, Vclxe2x88x921*, is at least 60 percent of the total cutting layer volume, Vcl.
Another embodiment discloses a cutter element for use in a drill bit comprising a substrate comprising a grip portion and an extension portion, the grip portion having an insert axis and the extension portion having a substrate apex. A superhard cutting layer is affixed to the extension portion so as to define an interface surface. The cutting layer has a chisel-shaped cutting surface and wherein the substrate and the cutting layer are shaped such that a plane that includes the insert axis divides the cutting layer such that the volume of the cutting layer on a first side of the plane is at least 60 percent of the total volume of the cutting layer.